Kirki_Color::adjust_brightness PHP Method

        public static function adjust_brightness($hex, $steps)
        {
            $hex = self::sanitize_hex($hex, false);
            $steps = max(-255, min(255, $steps));
            // Adjust number of steps and keep it inside 0 to 255.
            $red = max(0, min(255, hexdec(substr($hex, 0, 2)) + $steps));
            $green = max(0, min(255, hexdec(substr($hex, 2, 2)) + $steps));
            $blue = max(0, min(255, hexdec(substr($hex, 4, 2)) + $steps));
            $red_hex = str_pad(dechex($red), 2, '0', STR_PAD_LEFT);
            $green_hex = str_pad(dechex($green), 2, '0', STR_PAD_LEFT);
            $blue_hex = str_pad(dechex($blue), 2, '0', STR_PAD_LEFT);
            return self::sanitize_hex($red_hex . $green_hex . $blue_hex);
        }

 /**
  * Add custom CSS rules to the head, applying our custom styles
  */
 function custom_css()
 {
     $color = $this->get_admin_colors();
     $config = Kirki::config();
     $color_font = false;
     $color_accent = $config->get('color_accent', $color['icon_colors']['focus']);
     $color_back = $config->get('color_back', '#ffffff');
     $color_font = 170 > \Kirki_Color::get_brightness($color_back) ? '#f2f2f2' : '#222';
     $styles = '<style>';
     // Background styles
     $styles .= '#customize-controls .wp-full-overlay-sidebar-content{background-color:' . $color_back . ';}';
     $styles .= '#customize-theme-controls .accordion-section-title, #customize-info .accordion-section-title,#customize-info .accordion-section-title:hover,#customize-info.open .accordion-section-title{background-color:' . $color_back . ';color:' . $color_font . ';}';
     $styles .= '#customize-theme-controls .control-section .accordion-section-title:hover,#customize-theme-controls .control-section .accordion-section-title:focus,.control-section.control-panel>.accordion-section-title:after{background-color:' . \Kirki_Color::adjust_brightness($color_back, -10) . ';color:' . $color_font . ';}';
     $styles .= '#customize-theme-controls .control-section.control-panel>h3.accordion-section-title:focus:after, #customize-theme-controls .control-section.control-panel>h3.accordion-section-title:hover:after{background-color:' . \Kirki_Color::adjust_brightness($color_back, -20) . ';color:' . $color_font . ';}';
     $styles .= '#customize-theme-controls .control-section.open .accordion-section-title{background-color:' . $color_accent . ' !important;color:' . $color_font . ' !important;}';
     // Tooltip styles
     // $styles .= 'li.customize-control a.button.tooltip.hint--left {color:' . $color_accent . ';}';
     // Image-Radio styles
     $styles .= '.customize-control-radio-image .image.ui-buttonset label.ui-state-active {border-color:' . $color_accent . ';}';
     // Buttonset-Radio styles
     $styles .= '.customize-control-radio-buttonset label.ui-state-active{background-color:' . $color_accent . ';color:' . $color_font . ';}';
     // Slider Controls
     $styles .= '.customize-control-slider .ui-slider .ui-slider-handle{background-color:' . $color_accent . ';border-color:' . $color_accent . ';}';
     // Switch Controls
     $styles .= '.customize-control-switch .Switch .On, .customize-control-toggle .Switch .On{color:' . $color_accent . ';}';
     // Toggle Controls
     $styles .= '.customize-control-switch .Switch.Round.On, .customize-control-toggle .Switch.Round.On{background-color:' . \Kirki_Color::adjust_brightness($color_accent, -10) . ';}';
     // Sortable Controls
     $styles .= '.customize-control-sortable ul.ui-sortable li .dashicons.visibility{color:' . $color_accent . ';}';
     // Palette Controls
     $styles .= '.customize-control-palette label.ui-state-active.ui-button.ui-widget span.ui-button-text {border-color:' . $color_accent . ';}';
     $styles .= '</style>';
     echo $styles;
 }